This invention relates to optical mirrors and more particularly to a thermally compensated mirror having means for compensating tilt of the reflective surface.
Mirrors when subjected to intense beam of radiation are subject to extreme thermal loads resulting in thermal deformation of the mirror substrate with a corresponding distortion of the reflective surface. Many applications utilizing optical mirrors require that the optically reflective surface of the mirror maintain its desired configuration to within one-twentieth to one-one hundredth of the wavelength of the radiation incident thereon. Accordingly, the mirrors typically require cooling to maintain the stringent requirements on the reflective surface. Unfortunately, passing a coolant through the mirror results in stresses being generated within the mirror from the uneven pressure distribution of the coolant flowing therethrough and from the frictional heating of the coolant flowing through the coolant passages within the mirror.
Eitel in U.S. Pat. No. 4,110,013, filed Apr. 13, 1977 and held with the present application by a common assignee, discloses a metal cooled mirror having a faceplate assembly with a laminated structure comprising five plates with coolant channels disposed therein for passing coolant in heat exchanger relationship thereto. The laminated structure has means for passing a coolant therethrough in heat exchanger relationship therewith for minimizing the temperature increase of the mirror and the resulting thermal deformation thereto. Dunn et al. in U.S. Pat. No. 3,884,558, filed July 3, 1972 discloses a laminated mirror structure having a faceplate with grooves disposed therein adapted for providing a counterflow of coolant through alternate grooves therein. A second plate disposed adjacent the faceplate includes grooves disposed therein in a perpendicular relationship to the grooves disposed within the faceplate and adapted for passing coolant therethrough with counterflow of the coolant between adjacent coolant channels. Additionally, Sorenson et al. in U.S. Pat. No. 3,708,223 discloses an optical mirror having a faceplate with an optically reflective surface in heat exchanger relationship with coolant passages in which the flow of coolant in adjacent coolant passages is preferably countercurrent in order to cool the optically reflective surface in a thermally balanced manner. See also Griest in U.S. Pat. No. 3,781,094 and Dunn et al. in U.S. Pat. No. 3,817,800.
The prior art devices all suffer from the disadvantage that flowing coolant therethrough yields a temperature difference produced by absorbed and frictional heating across the width of the mirror resulting in a tilt distortion of the reflective surface of the mirror.